Change the topic of a message

Select the first option. It may be called View source / Edit topic,
or simply Edit. If it's called View source, then you are not
allowed to edit the topic of that message.

Edit the topic.

A dropdown with three options will appear to the right:
Change only this message topic, Change later messages to this topic, and
Change previous and following messages to this topic. Pick the appropriate
option.

Click Save.

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